The basic problem is that the raw sensor data probably is tilted that way after adjusting for white balance. So, the blown highlights end up with a pink/magenta cast. The solution is in the Highlight Reconstruction module, and in specific, the clipping setting — you tell Darktable to just discard the magenta highlight color information.
It can also be helpful to turn on one of the reconstruction modes, which help retain some detail rather than just clipping everything to flat gray. Here, I used Reconstruct in LCh, and turned the clipping way down:
… and voila! — goodbye, pink.
You can read more on this module in the darktable documentation. The module also has a blending option, which can be used to apply the feature to only a part of the image. I don’t think that’s necessary in this case, but it might be useful in others. (Read more on that in the manual, too, here.)